Air distribution control

ABSTRACT

An apparatus and method for limiting the flow in a variable air volume distribution system in which a sensed differential pressure provides a signal to increase or decrease air flow. Further circuitry is provided to allow the flow limiting means to properly function with seasonal changeovers.

United States Patent McNabney Sept. 12, 1972 AIR DISTRIBUTION CONTROL 2,236,914 4/1941 Nessell .236]! C UX 72 Inventor: John c. McNabney, La Crosse,Wis. iggz'ggg 31:31; i332 i g g a [73] Assignee: The Trane Company, La Crosse, 2598397 5/1952 Levine 'g Wis. [221 March 16, 1971 Primary Examiner-Edward J. Michael [2]] Appl. No.: 124,732 AttorneyArthur 0. Andersen, Carl M. Lewis and Robert E. Lowe Related US. Application Data [63] Continuation-impart of Set. N0. 86,661, Nov. 1 ABSTRACT 1970. An apparatus and method for limiting the flow in a variable air volume distribution system in which a [52] U.S.Cl. ..236I1C,236/49 sensed differential pressure provides a i l to i In. C.- ..F24f crease o decrease air flow Further circuitry is pro- Field sul'dl 23 vided to allow the flow limiting means to properly function with seasonal changeovers. [56] References Cited UNITED STATES PATENTS 7 Claims, 3 Drawing Figures 2,196,687 4/1940 Steinfield ..2 36li C UK 60 GI I k r i m 630 \sa I z A 5 H 1 Will? 52 62 K\65 PAIE'NTEDSEM m2 3.690.548

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ATTOR EY AIR DISTRIBUTION CONTROL CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of my copending application entitled, Flow Limiting Device," Ser. No. 86,661, filed Nov. 4,1970.

BACKGROUND OF THE INVENTION This invention relates to conditioned air distribution systems of the type wherein air which has been conditioned at a central source is distributed to a plurality of zones. These systems may operate in various manners. One type of system furnishes air at constant volume, the temperature of the air being varied in accordance with the demand from the conditioned space. Another system provides air at constant temperature,. and the volume of air delivered to a zone is varied in accordance with zone demands.

The latter system, which may be termed a variable air volume system, offers many advantages which flow from requiring only one source of conditioned air at constant temperature. Ductwork can be kept relatively simple, and control valves generally do not have to be duplicated as may be the case in a dual-duct, dual-air source system. The variable air volume system lends itself particularly well to a structure with a more or less consistent seasonal requirement for either heating or cooling, for example, the interior portions of an office building wherein the heat provided by lighting and occupants is sufficient to meet or exceed heating requirements, and a consistent requirement for cooling exists.

Such systems are not without problems, however, particularly in the area of control. It has been found, for example, that most air distributing units have an optimum operating range for purposes of efficiency and noise level. Accordingly, a zone which calls for an excessive amount of conditioned air may cause a sufficientlyhigh volume of air flow to create objectionable noise. Moreover, an excessive demand for conditioned air in one or more zones may give rise to an inbalance in the overall system resulting in an air deficiency in other zones. This problem can be effectively solved through the use of the flow limiting device and circuitry disclosed in my copending application, Ser. No. 86,661, filed Nov. 4, 1970, entitled Flow Limiting Device.

As is described in the aforementioned copending application, such a flow limiting device is normally configured to receive cooled air at all times or heated air at all times, depending upon the type of building in which the system is installed, the prevailing climatalogical conditions, and the like. However, some installations may require the capability for seasonal changeover wherein the system must be capable of delivering either heated or cooled air on signal.

SUMMARY OF THE INVENTION It is an object of the present invention therefor to provide a flow limiting device for variable air volume systems which is operable in response to system pressure.

It is a further object to provide such a flow limiting device which does not require the inducement of an artificial pressure drop in the air delivery system.

A still further object is to provide a device and method wherein air flow volume normally controlled thermostatically can also be controlled in response to system velocity pressure.

A further object is to provide an additional switching means in the flow limiting circuit to accomplish changeover from cooling to heating and vice versa.

The present invention has particular applicability to air distribution systems wherein air is delivered at substantially constant temperature in varying amounts to a plurality of zones. The desired temperature in each zone may be adjusted by varying the volume of conditioned air admitted to the'zone, which volume may be regulated by a thermostat. The volume of conditioned air admitted into the zone may be regulated by a single damper in the outlet box for that zone. When the thermostatic control indicates that additional conditioned air is required, the damper is driven further open. This action may continue in response to the thermostatic signal until the damper is sufficiently open that the air flow exceeds the design limitations. Should this occur, a pressure responsive means acting upon a measured differential pressure overrides the thermostatic control and drives the damper toward a closed position. When the damper has been sufficiently closed to bring the air flow within design parameters, the pressure responsive means is taken out of the control circuit and air flow control switches back to the temperature responsive thermostat. When the air system is required to furnish either heated or cooled air for seasonal changes, an additional temperature sensor and switching means may be provided to reverse the effects of the thermostatic control.

DESCRIPTION OF PREFERRED EMBODIMENTS Embodiments of the present invention will be described in greater detail with reference to the attached drawings in which:

FIG. 1 is a schematic view of an air terminal unit with related control circuitry,

FIG. 2 is a circuit diagram of the circuit of FIG. 1 including a seasonal switchover means, and

FIG. 3 is a control circuit diagram illustrating another embodiment of the circuit shown in FIG. 1.

Referring to the drawings, an air terminal or outlet box is shown generally at 10, having an air inlet portion 11 and an air outlet portion 12. Inlet ll communicates with a source of air, indicated by the arrow therein, and outlet 12 communicates with a zone to receive conditioned air from terminal box 10 through outlet portion 12. The conditioned air passing through the terminal box 10 is preferably of substantially constant temperature, which temperature may be cooler or warmer than the unconditioned temperature within the zone.

Mounted within terminal box 10 is a single bladed damper 13 which is adapted to rotate about rod 14 passing between the sidewalls of terminal box 10. The rotation of damper 13 may be effected through lever arm 15 attached to damper l3 and rod 16. Rod 16 may be connected to common shaft 18 of damper closing motor 19 and damper opening motor 20 through a threaded coupling 17 held in bracket 22. In this manner the rotation of the shaft 18 produces sliding linear motion of coupling 17 and shaft 16, which in turn produces rotational motion of lever arm 15 and damper l3.

If desired, damper stops 21 may be provided along the top and bottom walls of terminal box to allow the damper 13 to come to a securely closed position within the terminal box. The face of damper stops 21 may be sloped at an appropriate angle to cooperate with damper 12 so that damper 13 may lie flat against the stops in the closed position.

Disposed within inlet portion 11 of terminal box 10 are sensing probes 30 and 31. Probe 30 is arranged facing directly upstream of the air flow and probe 31 is arranged within the airstream such that its orifice is perpendicular to the direction of air flow. Probes 30 and 31 have tubular passageways whereby the pressures sensed by each within the airstream are communicated to a pressure responsive means 32. The pressure responsive means 32 comprises an upper chamber 33 and a lower chamber 34 separated by a diaphragm 35. The flow from probe 30 is fed into lower chamber 34, while the flow from probe 31 is fed into upper chamber 33. It has been found that probe 30 oriented upstream of the flow will measure a pressure equivalent of velocity pressure plus static pressure in inlet 11, whereas probe 31 oriented perpendicular to the flow will indicate a pressure equivalent to the static pressure minus velocity pressure. It can be seen that the differential pressure across diaphragm 35 will then be equal to twice' the velocity pressure at inlet 11. Moreover, the probes measure system pressures directly, and require no artificial pressure drop for their operation.

The movement of diaphragm 35 is translated to snapaction switch 38, which carries common terminal 40, normally closed terminal 41 and normally open terminal 42. The internal operation of pressure responsive means 32 is more fully described in my copending application referred to herein before.

Power to operate damper motors 19 and 20 is provided by power source 50, which may be building line voltage, the voltage being stepped down by transformer 51. Completing the control circuit is thermostat 52 the operation of which will be explained in more detail hereinafter.

In order to give the control circuit the capability of controlling the flow of either cooled or heated air, coacting snap-action switches'60 and 61 are provided. Switch 60 carries common terminal 62, normally closed terminal 63 and normally open terminal 64. Switch 61 carries similar terminals, specifically common terminal 65, normally closed terminal 66, and normally open terminal 67. The terminals of switches 60 and 61 are actuated to the open and closed positions in response to the temperature sensed at bulb 68.

Under normal conditions it is desirable to operate the present system in response to the temperature in the conditioned zone. In operation, air is conditioned at a central source and is distributed through a duct network to terminal boxes, such as terminal box 10, from which it flows into the zone to be conditioned. The air issupplied at substantially constant temperature, and the temperature of the zone is then adjusted by adjusting the volume of conditioned air delivered to that zone. For purposes of the following illustration, it will be assumed that the conditioned air supplied has been cooled at a central point and is provided to the zone at a temperature below that within the zone.

One side of transformer 51 is electrically connected to common terminal 40 of snap-action switch 38, while the other side of transformer 51 is connected to closing motor 19 and opening motor 20. Normally closed contact 41 of switch 38 is connected to the bimetal element of thermostat 52. The low side, or falling temperature side of thermostat 52 is connected to damper closing motor 19, and the high side, or temperature rising side of thermostat 52 is connected to damper opening motor 20. With this circuitry, it can be seen that the damper motors 19 and 20 are directly controlled by thermostat 52.

As the temperature in the zone falls, the low side contact of thermostat 52 closes the circuit to the damper closing motor 19, driving the damper toward a closed position and reducing the supply of cooled air entering the zone. As the temperature in the zone rises, the bimetal contact of the thermostat moves away from the low side contact, opening the circuit to the damper closing motor 19 and closing the circuit to the damper opening motor 20. This action reverses the rotation of shaft 19 and moves the damper toward an open position, thereby admitting a greater quantity of cooled air to terminal box 10 and to the conditioned zone through outlet 12. The damper motors l9 and 20 are preferably of the type having a very slow rate of rotation so a time on the order of several minutes is required to move the damper from the open position to the closed position.

Various conditions may occur within the building in which the air system is installed which may result in damper 13 being driven open to a position which allows a volume of air flow which exceeds the desired limits. Such a condition may occur during unusually high heat loads, system inbalance and the like. This condition will be sensed by probes 30 and 31 and transmitted to pressure responsive device 32. As the volume of air flowing through inlet 11 increases, the pressure in lower chamber 34 increases and the pressure in upper chamber 33 decreases proportionately. The pressure differential causes diaphragm 35 to be displaced upwardly, causing normally closed contact 41 to be opened and normally opened contact 42 to be closed.

Removing contact 41 from the circuit immediately removes thermostat 52from the circuit to the damper motors. At the same time, contact 42 is closed, providing power directly to the damper closing motor 19 and driving damper 13 toward a closed position. This action will continue until damper 13 is sufficiently closed to reduce the air flow through inlet 11 to a point whereby diaphragm 35 recedes toward its neutral position due to a decrease in differential pressure as sensed by probes 30 and 31. As diaphragm 35 recedes, contact 42 is opened and normally closed contact 41 again becomes closed. This discontinues power to the damper closing motor 19 and brings thermostat 52 back into the circuit.

The flow limiting device in circuit will generally come into play when a zone encounters a greater than normal heat load requiring an unusually high demand for cooling. It is generally during such periods that the flow limiting circuit will be necessary to override the thermostatic circuit in order to maintain air flow within desired limits. As has been pointed out, when the flow limiting circuit is energized, it immediately supplies power to the damper closing motor 19, even though the thermostat is calling for cooling. The flow limiting circuit remains energized until the flow volume through inlet 11 has been reduced within design operating limits, at which time contact shifts from terminal 42 to terminal 41 of switch 38, thereby de-energizing the flow limiting circuit and re-energizing the thermostatic circuit.

While the circuit and control means described with reference to FIG. 1 is effective in controlling air flow under a constant heating or cooling demand, it can be seen that the circuit would not operable in an installation wherein a seasonal or other switchover from heating to cooling was required. For example, it can be seen if heated rather than cooled air were being supplied through inlet 11 to terminal box and ultimately to the conditioned space through outlet 12, an increase in zone temperature would cause the thermostat bimetal contact to move to the right to the high side contact and to close the circuit to damper opening motor20. This, of course, would result in an even greater quantity of heated air being introduced into the zone which would in turn cause further rise in temperature, and keep the bimetal contact closed on the high side contact.

In order to correct this situation and provide for a seasonal changeover capability, the additional switches shown in FIG. 2 are provided. In that embodiment, the high side of the thermostat is connected to the terminal of switch 60 and the low side of the thermostat is connected to the common terminal of switch 61. The terminal 63 of switch 60 is connected to terminal 67 of switch 61 which is then connected to the damper opening motor 20. The normally closed terminal 66 of switch 61 is connected to normally open terminal 64 of switch 60, which is in turn connected to one side of damper closing motor 19 and to normally open terminal 42 of switch 38.

The designation normally opened as applied to terminal 64 of switch 60 and terminal 67 of switch 61 respectively indicates their electrical configuration with regard to common terminal 62 and common terminal 65 respectively. The opening and closing of contacts 63, 64, 66, and 67 occurs in response to a temperature sensed by bulb 68 which may be located in the air supply duct or inlet. Switches 60 and 61 therefor will be selected to operate at a given set point, wherein contact 63 and 66 are always closed above a certain temperature, and terminal 64 and 67 become closed when the temperature falls below that preselected set point. Bulb 68 then merely senses the temperature and provides switches 60 and 61 with the necessary input to cause the opening and closing of the aforementioned terminals.

In order to more fully describe. the operation of this embodiment, it will be assumed that the set point of switches 60 and 61 is 65 F. and that the system is operating during the cooling season wherein the normal, unconditioned temperature of the zone is well above the 65 F. set point. When the temperature in the zone rises, and the bimetal contact of thermostat will move to the right to the high side contact where it will close with the high side contact, completing circuit through common terminal 62, normally closed terminal 63, normally open terminal 67 of switch 61, and ultimately damper opening motor 20. It will be seen that this action then will drive damper 13 toward a more open position thereby admitting a greater quantity of cooled air to the zone. Should the temperature in the zone fall, the bimetal contact of the thermostat will move to the left making contact with the low side contact, thereby completing a circuit to the common terminal 65 of switch 61, through normally closed terminal 66 to normally open terminal 64 of switch and eventually with damper closing motor 19. This action will drive damper 13 toward amore closed position and reduce the volume of cooled air being supplied to the zone.

With a seasonal changeover, the central source of conditioned air will now be equipped to supply heated instead of cooled air to all zones. When the temperature of air in the inlet duct rises above the preset point of F. as sensed by bulb 68, switches 60 and 61 are actuated whereby normally opened terminals 63 and 66 become opened, producing a circuit as indicated by the dashed lines. Now as the temperature in the zone rises the thermostat again will move to the right making contact with the high side contact, thereby completing a circuit to a common terminal 62 of switch 60 and normally open terminal 64 to the closed motor 19. This action will cause damper 13 to move toward a more closed position thereby reducing the volume of heated air being supplied to the zone.

When the temperature in the zone begins to fall, the contact of the thermostat will move to the left completing the circuit to common terminal 65 of switch 61 through normally opened contact 67 to damper opening motor 20. This action will drive the damper 13 toward a more open position thereby admitting greater quantities of heated air to the zone.

It will be noted that pressure responsive means 32 maintains an identical function regardless of the addition of the seasonal switchover means. That is, in the event that the volume flow through inlet 1 1 exceeds the design maximum, normally closed terminal 41 of switch 38 will open and the normally opened terminal 42 will close thereby supplying power directly to damper closed motor 19 and driving the damper toward a more closed position, regardless of whether cooled or heated air is being supplied to the conditioned zone and regardless of the demand for heating or cooling on the part of the thermostat.

An additional feature of the invention can be described with particular reference to FIG. 3. For this example, it will be assumed again that the zone is calling for cooled air at all times. When the demand for cooling in the zone is sufficient to actuate pressure responsive means 32 to override the thermostatic circuit, damper closing motor 19 will be actuated to drive damper 13 toward a closed position as has been described in detail herein before. When damper 13 is sufficiently closed to reduce the air flow through inlet moved toward a more open position. Should this continue for an extended period of time, the air flow may once again exceed the desired limitations.

Thus it can be seen that it is possible under certain conditions for the system to constantly cycle between the thermostatic control circuitry and the pressure sensing override circuit with the damper motors being cycled on and off. Although this situation would eventually correct itself as the temperature in the zone was reduced, it can be prevented or greatly alleviated in the embodiment of the present invention shown in FIG. 3.

In that embodiment, the basic air distributing apparatus remains the same and the control circuit is somewhat modified to provide for an additional snapaction switch 39. The common terminal of snap-action switch 38 is connected to the high side terminal of thermostat 52. The normally closed terminal 41 connects to one side-of the damper opening motor 20. The common terminal of snap-action switch 39 is connected to the power supply transformer and the bimetal elements of thermostat 52. The normally open terminal N of switch 39 is connected to one side of damper closing motor 19 and to the low side contact of thermostat 52.

In this embodiment, as the temperature in the zone rises, the bimetal contact of thermostat 52 moves to the right and contacts the high side terminal. This energizes the circuit to the damper opening motor by way of common terminal 40 and terminal 41 of switch 38. In this manner, the damper 13 is driven to a more open position in order to admit greater quantities of cool air.

When the air flow in the duct 11 exceeds the preset maximum, terminal 42 becomes closed and terminal 41 becomes opened. This opens the circuit to damper opening motor 20 and terminates the opening movement of the damper, allowing the damper to remain at its preset maximum opening.

In the event that the air flow through duct 11 continues to rise, such as may result from an excessive number of units in the overall system having been shut down, terminal NC of switch 39 will be opened and terminal N0 of switch 39 will be closed. This then supplies power directly to damper closing motor 19, energizing that motor and driving the damper towards a closed position. This action continues until such time as the air flow through the duct is reduced to a predetermined acceptable level. At that time, terminal N0 of switch 39 opens, opening the circuit thereby stopping the damper. The damper opening motor 20 remains out of this circuit without a further drop in flow, however, the damper closing motor 19 is now connected to the low side terminal of thermostat 52 so that in the event that the temperature in the zone falls damper closing motor 19 will be energized, directly driving the damper towards a more closed position.

With a further drop in air flow through duct 1 I, normally closed terminal NC of switch 39 opens and terminal 41 of switch 38 is closed. This action returns the high side terminal of thermostat 52 to the circuit as well as the low side terminal of the thermostat. Either the damper opening motor 20 or the damper closing motor 19 can now operate in direct response to a rise or fall in temperature respectively.

' It is preferred to preset the operating point of switches 38 and 39 to compliment one another and to provide a range of air flow within which the switches operate. For example, in the system which is designed to operate using air flow rates in the order of approximately 300 cubic feet per minute, switch 39 might be set to cut into the circuit at 300 CFM and cut out at 270 CFM, whereas switch 39 would cut in at 330 CFM and cut out at 300 CFM. These figures are, of course, intended to be by way of example only and in no way intended to be a limitation on the invention. It can be seen that through the addition of switch 39 and the composition of the circuit as described, the possibility of the hunting situation which may'arise in the embodiment shown in FIG. 1 is effectively eliminated.

While in the foregoing specification the invention has been described in considerable detail, it will be understood that such detail is for the purposes of illustration and description, and not for the purpose of limitation of the invention which is found in the appended claims. It will be understood that modifications and variations in the invention can be made by those skilled in the art without departing from the spirit or scope of the invention. I

I claim:

1. An air distribution system for delivering conditioned air at a preselected substantially constant temperature to a plurality of zones wherein the temperature of each of said zones is controlled by varying the volume of conditioned air delivered to said zone, said system including ducts for carrying conditioned air from a source to at least one air terminal unit for delivering conditioned air to at least one of said zones, said unit including air throttling means for varying the volume of conditioned air delivered to said zone, said throttling means being normally responsive to a zone thermostat whereby said throttling means is caused to supply an increased volume of conditioned air in response to a first position of said thermostat, a duct temperature sensor, temperature responsive switch means in operative communication with said zone thermostat and said throttling means and responsive to said duct temperature sensor whereby said throttling means is caused to supply a decreased volume of conditioned air in response to said first position of said thermostat when the duct temperature rises above a preselected temperature; means for sensing variations in the rate of air flow into said unit, pressure responsive means communicating with said sensing means and movable in response to changes in said rate of air flow, switch means actuated in response to the movement of said pressure responsive means to override said zone thermostat and said temperature responsive switch means when said rate of air flow exceeds a predetermined rate.

2. The apparatus according to claim 1 wherein said air throttling means includes a motor driven blade damper.

3. The apparatus according to claim 2 including two coaxially mounted electric motors, one of said motors driving said damper in a first direction and the other of said motors driving said damper in the opposite direction.

4. The apparatus according to claim 1 wherein said temperature responsive switch means includes a pair of interconnected snapaction switches in operative communication with said duct temperature sensor.

5. The apparatus according to claim 4 wherein each of said snap-action switches have a common contact, a normally closed contac t, and a normally open contact, said normally closed contact of each switch being electrically connected to the normally open contact of the other switch.

6. The apparatus according to claim 5 wherein said normally closed contacts become open and said normally open contacts become closed at a predetermined temperature as sensed by said duct temperature sensor.

7. An air distribution system comprising in combination:

a. a central source of conditioned air;

b. duct means connecting a plurality of zones with said central source;

c. at least one air terminal box communicating with each of said zones;

d. damper means in at least one of said terminal boxes, said damper means being openable and closeable to vary the volume of air passing from said terminal box to said zone;

e. motor means drivingly connected to said damper means to open and close said damper means;

f. temperature responsive means in said zone to control said motor means wherein said motor means normally drives said damper means toward an opened position upon an increase in temperature in said zone;

g. switch means operably connecting said motor means and said temperature responsive means and including duct temperature sensing means, said switch means causing said motor means to drive said damper means toward an opened position upon a decrease in temperature in said zone when said duct temperature sensing means senses a temperature above a preset limit;

h. means for sensing the air flow rate into said terminal box,

i. said sensing means communicating with pressure responsive means movable in response to changes in said air flow rate to override said temperature responsive means when said air flow rate exceeds a predetermined maximum rate. 

1. An air distribution system for delivering conditioned air at a preselected substantially constant temperature to a plurality of zones wherein the temperature of each of said zones is controlled by varying the volume of conditioned air delivered to said zone, said system including ducts for carrying conditioned air from a source to at least one air terminal unit for delivering conditioned air to at least one of said zones, said unit including air throttling means for varying the volume of conditioned air delivered to said zone, said throttling means being normally responsive to a zone thermostat whereby said throttling means is caused to supply an increased volume of conditioned air in response to a first position of said thermostat, a duct temperature sensor, temperature responsive switch means in operative communication with said zone thermostat and said throttling means and responsive to said duct temperature sensor whereby said throttling means is caused to supply a decreased volume of conditioned air in response to said first position of said thermostat when the duct temperature rises above a preselected temperature; means for sensing variations in the rate of air flow into said unit, pressure responsive means communicating with said sensing means and movable in response to changes in said rate of air flow, switch means actuated in response to the movement of said pressure responsive means to override said zone thermostat and said temperature responsive switch means when said rate of air flow exceeds a predetermined rate.
 2. The apparatus according to claim 1 wherein said air throttling means includes a motor driven blade damper.
 3. The apparatus according to claim 2 including two coaxially mounted electric motors, one of said motors driving said damper in a first direction and the other of said motors driving said damper in the opposite direction.
 4. The apparatus according to claim 1 wherein said temperature responsive switch means includes a pair of interconnected snap-action switches in operative communication with said duct temperature sensor.
 5. The apparatus according to claim 4 wherein each of said snap-action switches have a common contact, a normally closed contact, and a normally open contact, said normally closed contact of each switch being electrically connected to the normally open contact of the other switch.
 6. The apparatus according to claim 5 wherein said normally closed contacts become open and said normally open contacts become closed at a predetermined temperature as sensed by said duct temperature sensor.
 7. An air distribution system comprising in combination: a. a central source of conditioned air; b. duct means connecting a plurality of zones with said central source; c. at least one air terminal box communicating with each of said zones; d. damper means in at least one of said terminal boxes, said damper means being openable and closeable to vary the volume of air passing from said terminal box to said zone; e. motor means drivingly connected to said damper means to open and close said damper means; f. temperature responsive means in said zone to control said motor means wherein said motor means normally drives said damper means toward an opened position upon an increase in temperature in said zone; g. switch means operably connecting said motor means and said temperature responsive means and including duct temperature sensing means, said switch means causing said motor means to drive said damper means toward an opened position upon a decrease in temperature in said zone when said duct temperature sensing means senses a temperature above a preset limit; h. means for sensing the air flow rate into said terminal box, i. said sensing means communicating with pressure responsive means movable in response to changes in said air flow rate to override said temperature responsive means when said air flow rate exceeds a predetermined maximum rate. 